Spunbonded olefin sheets, such as those commercially available under the registered trademark "TYVEK.RTM." from DuPont, Wilmington, Del., have gained use for various types of packaging. Spunbonded olefin sheets are tough, durable sheets of high density polyethylene fibers. The sheets are formed by flash spinning continuous strands of ultrafine interconnected fibers, dispersing them onto a moving belt and then bonding them together with heat and pressure into a multi-layer sheet. The combination of polymer and method of manufacture account for the ability of spunbonded olefin sheet material to be highly resistant to penetration by contaminating microorganisms, yet capable of allowing quick passage of sterilizing gas and accompanying moisture vapor. This combination of properties makes spunbonded olefin a particularly useful material for packaging medical supplies or devices which must be sterilized and maintained in sterile packaging until used. However, one drawback to the use of spunbonded olefin sheets for sterile medical packaging is that it has been believed that the material should not be subjected to package forming processes which would stretch or thermally form the material since to do so would cause holes to form in the spunbonded olefin sheets, thus rendering the sheets no longer resistant to contamination by microorganisms.